Optical opacity, for example, "hiding" of a paint film, is a property which is of great importance in the coating industry where such films are employed to decorate and protect the substrate on which they are applied. In many such applications, such as those in the appliance industry, white finishes of high hiding and having good protective qualities are desired, and special effort has been expended in order to produce suitable coatings of this type.
The optical opacity of films is achieved either by absorption of the incident light or by scattering of the incident light or a combination of these two. Thus, black is opaque because it absorbs the light incident on it and white is opaque because it back-scatters the incident light. Light is either absorbed or scattered before it can reach the substrate. The ideal white coating, therefore, is one which has zero absorption and maximum scattering.
Opaque films have conventionally been prepared by adding pigment, such as titanium dioxide, to a film-forming material which would otherwise be colorless or transparent when cast in a film. The necessity for adding an opacifying agent obviously increases the cost of the resultant film. Moreover, the addition of such materials often causes the impairment of the physical properties of the resultant film.
More recently, processes have been described in the art for preparing opaque films which rely for opacity upon the presence of a large number of cells or voids in the film. One such method involves depositing a film from an emulsion, such as an oil-in-water or a water-in-oil emulsion. Another technique for obtaining such cellular films is by preparing an aqueous dispersion of a film-forming polymer containing a water-soluble organic solvent in an amount which is insufficient to dissolve the polymer. This aqueous dispersion is then cast as a film and water is evaporated, thereby causing entrapment of minute droplets of the organic solvent in the polymer. The film is then washed to dissolve the entrapped minute droplets of solvent and the film is dried.
Opaque films having such microscopic voids display improved hiding power over films utilizing pigments and, moreover, are less costly and possess improved physical properties. But despite the over-all utility of films produced according to this method, their use has been handicapped by the difficulties inherent in the preparation of the film-forming composition. When an emulsion technique is employed, for example, care must be taken in order to insure its stability, i.e., so that it will not break before it is used to deposit a film. This frequently requires the use of emulsifying agents. However, emulsifying agents which are then present in the film detract from the physical properties of the film, such as its water repellency, scrub resistance, etc.
More generally, the formation of microvoids according to conventional practice usually involves a spontaneous and non-controllable process. Consequently, wide variations in the size of the voids and their distribution in the film-forming material often result, with an accompanying decrease in both opacity and film characteristics.
In brief, it has not been possible according to any known procedures to produce an opaque, non-pigmented coating or film without employing relatively elaborate film-forming methods, as with the above-described techniques, or without foregoing desirable film characteristics, as has usually occurred with the use of pigments.